The present invention relates to equipment for diagnostic and therapeutic radiology and methods of making the same and, more particularly, to methods for exhausting x-ray tubes during the manufacturing process.
Recently, it has been found that the internal vacuum obtained in the x-ray tube envelope has been only been about 1.times.10.sup.-5 torrs. This internal vacuum has allowed "spitting" which occurs when the electrical path of the electron beam is diverted to some other point in the vacuum space rather than the focal track of the x-ray tube target. Spitting occurs because there are more particles left in the vacuum space that can attract the electrons being generated. Additionally, the manufacturing process called "exhaust" presently requires up to thirty hours to complete, which is entirely too long in the manufacturing process.
Current manufacturing "exhaust" practice utilizes a small about 1/2" to about 3/4" inside diameter tubulation connected to a turbomolecular pump having a pumping speed of approximately 1 liter per second as measured at the target. As is known, pumping speed or conductance is directly related to the inside diameter of the pumping port or tubulation. While length of the tube which does have an effect, it is much less than the effect of the diameter.
During x-ray tube manufacturing, the exhaust port of the envelope/tubulation connection of the x-ray tube is sealed off after evacuation by standard glass blowing technique of thermal collapse, fusion and separation of the small diameter (1 to 2 cm inside diameter) exhaust tubulation. The lowest pressure that can be achieved with the current configuration is limited by the conductance of the exhaust tubulation. The conductance (c) of this tube is proportional to the diameter (d) and to the length (I): EQU c.about.d.sup.3 /I [1]
To achieve lower pressures, the conductance must be increased. To increase the conductance, a larger diameter exhaust tubulation must be used.
Post "exhaust" process inspection has revealed that the current method may be insufficient to provide effective removal of the gases evolved during the exhaust process and thereby leave the x-ray tube enclosure with a high pressure condition which in turn has been related to early failure of the assembly in the field. The "exhaust" process method has not been changed to a larger diameter pumping port or tubulation because of the past inability to effectively seal the envelope/tubulation connection after the completion of the "exhaust" process step.
The seal-off configuration currently used does not work with larger diameter tubulations, The "thermal collapse" phase becomes extremely unstable and the tubulation buckles in an uncontrollable fashion. Effective "fusion" of the buckled tubulation is not possible with this prior configuration.
Due to unacceptable failures after seasoning and prior to being shipped, the need for an improved x-ray tube having an envelope evacuated to about 1.times.10.sup.-5 torr that would reduce or possibly eliminate the spitting while shortening the manufacturing cycle became apparent. Such an x-ray tube would have the exhaust process or a combination exhaust and seasoning process of the manufacturing process effective to evacuate the x-ray tube envelope to greater than about 1.times.10.sup.-5 torr, reducing the particles left in the vacuum space that could attract the electrons being generated such that failure due to "spitting" , which occurs when the electrical path of the electron beam is diverted to some other point in the vacuum space rather than the focal track of the target, should be significantly reduced, if not eliminated and reduce the about thirty (30) hours presently required to complete the exhaust process step.